How long is a "moon"?

All we officially know is that a moon is an arbitrary amount of time which according to Jayson Thiessen has no Earth equivalent. But is that really true?

Granting that a mysterious time scale can be used without much fear in storytelling as there's little chance of continuity error, there is precisely one reference in MLP canon that corresponds to an Earth equivalent, and it's likely the most innocuous comment one can imagine.

But before we get to the reference and proof, here are some other mentions of the "moons" measurement which have either a hard number or a reasonable path to estimation:

Apple Family Reunion: occurs every 100 moons.
The Gift of the Maud Pie: a Manehattan store is closed for "a few moons" due to the owner's vacation.
Every Little Thing She Does: 80 moons worth of scrap-booking photos.
Equestria Girls: The Crystal Mirror activates every 30 moons for 3 days.
Time between EQG1 and EQG2: 6 moons (as stated by Meghan McCarthy).

First, let's apply some general logic. A family reunion on Earth typically occurs every year or every other year. Given ponies' highly social nature, we can expect similar frequency in Equestria.

The average duration of a vacation is one to two weeks. Terminating one's primary source of income for an excessive amount of time, as in the case of a business owner shutting down his or her store, means we cannot reasonably expect the Manehattan store to be closed more than a month.

Again, due to the highly social nature of ponies, one cannot reasonably expect Applejack to delay a scrapbook for many years. It also stands to reason that the scrapbook is a work in progress, and the tasks in the episode are catching up after events in the series.

The Crystal Mirror is impossible to gauge without further information, but because a great many stories depend on its timing rules and because we have hard numbers, I included it.

We know from the beginning of Rainbow Rocks that the Dazzlings saw Sunset Shimmer being defeated, and they were nearby. How many months did they wait before acting on that information? It certainly shouldn't have taken long to transfer into Canterlot High if we're to account for any kind of continuity given that Twilight Sparkle simply walked from the portal and through the front door in Equestria Girls. Further, Sunset Shimmer clearly has not had much time to sway the student body in her favor, and not for lack of trying. It's safe to say that the time between Equestria Girls and Rainbow Rocks is short.

The Answer?

Now for the fun part. In season 4, Pinkie Apple Pie, a quick and easily missed statement of trivia from Granny Smith actually gives us both a hard number and a potential Earth equivalent.

"A Ponyville snail can hibernate for up to forty-eight moons!"

Snails can hibernate on Earth as well, typically in the winter months or during excessively dry summers. Snails in desert locales can hibernate for years until the environment is more suitable, but I think it's safe to say that Ponyville is not an arid environment. Therefore, we can comfortably say that they are land snails with a more moderate winter hibernation; say from October to April for a solid 6 months.

If 48 moons is 6 months, then there are 8 Equestrian moons for each Earth month. This is 2 moons per week, and given a 168 hour long week, the Equestrian moon would be 84 Earth hours, or 3.5 days. That's quite a bit shorter than the usual fan fiction estimate of ~30 days, and would give us some very reasonable estimates for the above situations:

Apple Family Reunion: occurs every 350 days, or approximately 1 year. Very reasonable, and corresponds to Earth practice.
The Gift of the Maud Pie: the Manehattan store is closed for about 10 days, or 2 business weeks (assuming "few" means 3). Very reasonable, and corresponds to what one would expect for the duration of your average vacation.
Every Little Thing She Does: 280 days, or ~9 months worth of scrap-booking photos. Again, very reasonable, especially if the backlog is due to the events of the series thus far.
Equestria Girls: The Crystal Mirror activates every 105 days, or 3.45 months, for 3 days. Almost three and a half months is a long time to be stuck in an alternate world while holding the key to your world's greatest weapon.
Time between EQG1 and EQG2: 21 days, or less than a month. Noting the events at the start of Rainbow Rocks, it's safe to say the Dazzlings moved quickly rather than waiting several months. Still an oddly long time, to be fair.

Obviously precision in the numbers is not currently possible. That said, when taking all of the evidence into account, the duration of a "moon" cannot be reconciled to more than a handful of days, either in Equestria or Earth. Anything longer raises difficult questions on the references where we have a hard number. For those references that are softer (thousands, hundreds, etc.) not much can be done other than debate "how many" in a rational manner.

Comments

[Seirs]

Well that certainly is the most reasonable rundown of this topic I've seen to date. nice.